1. Field of the Invention
The present invention relates to a manufacturing method of a rare-earth magnet for manufacturing a rare-earth magnet by performing hot plastic working on a sintered body.
2. Description of Related Art
A rare-earth magnet using a rare-earth element such as lanthanoid is also called a permanent magnet. The permanent magnet is used for driving motors of a hybrid vehicle, an electric vehicle, and the like, as well as motors constituting a hard disk and an MRI.
As an index of magnetic performance of the rare-earth magnet, residual magnetization (residual magnetic flux density) and coercive force are known. In regard to an increase in heat generation amount due to downsizing and high current density of a motor, a demand of heat resistance to a rare-earth magnet to be used is increased still more, and how magnetic characteristics of a magnet can be maintained under high-temperature use is one of important research themes in the technical field.
In the technical field, there has been known a method of manufacturing a rare-earth magnet (an oriented magnet) in such a manner that a sintered body is manufactured by performing pressing on a fine powder obtained by immediately solidifying Nd—Fe—B molten metal, for example, and hot plastic working is performed on the sintered body so as to give magnetic anisotropy thereto. Further, Japanese Patent Application Publication No. 2-138706 (JP 2-138706 A) describes an anisotropic permanent magnet configured such that hot plastic working is performed on a sintered body so as to improve anisotropy and to increase a residual magnetic flux density.
As the hot plastic working, hot upsetting has been known. In the hot upsetting, a plastic working mold constituted by a lower die, a side die, and an upper die (also referred to as a punch) slidable within the side die. Then, the sintered body put in a cavity of the plastic working mold is pressed by the upper die in a short time of around one second or less, for example, while being heated to achieve a predetermined processing rate.
Although magnetic anisotropy can be given to the sintered body by the hot upsetting, the sintered body is about to be deformed laterally due to the pressing by the upper die in the upsetting. It is known that, at the time when the sintered body is about to be deformed laterally, the sintered body receives, from the upper die and the lower die, a shearing frictional force in a direction opposite to a direction of the deformation. The shearing frictional force will be described in detail with reference to FIG. 18.
FIG. 18A illustrates an analytic model of a compact sandwiched between the upper die and the lower die before the upsetting. This analytic model is a group indicative of a compact constituted by many constituent cells to execute a finite element analysis by a computer. FIG. 18B illustrates a deforming state of the analytic model after the upsetting at a processing rate of 50%. In regard to the analytic model illustrated herein, since left and right sides of a sintered body show the same analysis result, only a right section is modelled.
When the sintered body is pressed by the upper die as illustrated in FIG. 18A, a free end surface of the sintered body that has no restriction is deformed laterally as illustrated in FIG. 18B. At the time of the lateral deformation, a top face and a bottom face of the sintered body receive shearing frictional forces opposite to a lateral deformation direction, from the upper die and the lower die, respectively. As a result, plastic deformation is promoted in a central region of the sintered body as compared with its peripheral region, so that the central region becomes a high strain region. This causes orientation disturbance in a crystal structure, which may cause a decrease in residual magnetization. Further, a yield of materials may decrease so that a manufacturing cost may increase.